In a mobile communication system like land mobile radiotelephone a zone which permits a mobile station to communicate with one base station is limited by the radio wave transmitting power used; therefore, in the case of implementing a wide service area, a number of zones Z1, Z2, ... including base stations B1, B2, ..., respectively, are provided, for example, as shown in FIG. 1, in such a manner as to permit communication from any zone via its base station and an exchange 20. Adjacent zones use signals of different frequencies to prevent interference between signals of different zones. In such a mobile communication system, when a certain mobile station 10 moves from the zone Z1 to the adjoining one Z2 while maintaining communication, the radio channel frequency used must be switched from F1 to F2 and during this hand off the communication is temporarily interrupted; this is a well-known problem in the art.
In recent years there has been under study the introduction of a digital mobile communication system employing TDMA (Time Division Multiple Access) techniques. In the digital mobile communication system utilizing the TDMA techniques each mobile station holds communication in each zone, using a designated time slot of a frequency specified by the base station in that zone. A communication channel specified in terms of frequency and time slot will hereinafter be referred to as a radio channel, but in the case where it is not intended to specify any particular time slot in the following description, the channel will be also called a frequency channel. Also in this instance, when the mobile station moves from a certain zone to an adjacent one, the radio channel must be switched accordingly. It is easily predictable that the switching of the radio channel will cause a momentary interruption of communication also in such a well-known digital mobile communication system which utilizes the TDMA techniques.
FIG. 2 is a timing chart for explaining the radio channel hand off procedure in the digital mobile communication system employing the TDMA techniques and the momentary interruption of communication which results from the channel hand off. Now, let it be assumed that the mobile station 10 moves from the zone Z1 to the zone Z2 in FIG. 1. Row A in FIG. 2 shows a signal of a frequency F1 transmitted from the base station B1. The transmitted signal is a recurrence of a TDMA frame FR which is composed of a predetermined number of, for instance, three time slots S1, S2 and S3, and for communication with the mobile station 10 in the zone, one time slot, the first time slot S1 in this example, is used. Row B in FIG. 2 shows a signal of a frequency F2 being transmitted from the base station B2 of the zone Z2 into which the mobile station 10 is to enter. Assume that the mobile station 10 uses an idle third time slot S3 of the transmitted signal from the base station B2 in the zone Z2. Row C shows a signal received by the mobile station 10 and row D the frequency of the received signal. Row E shows a signal transmitted from the mobile station 10 and row F a signal received by the base station B2 in the zone Z2.
When the reception at the frequency F1 gets worse as the mobile station 10 approaches the zone Z2, the base station B1 specifies the radio channel to be used, i.e. the frequency F2 and the time slot S3, through a signal A1 of the first slot S1 of the frame FR1 shown in row A of FIG. 2. Upon reception of such a radio channel specify signal, the mobile station 10 switches the receiving frequency to F2 as depicted in row D of FIG. 2, receives a signal of the frequency F2 from the base station B2, shown in row B of FIG. 2, and then establishes synchronization with a signal B1 of the third slot S3 of the frame FR1. This ensures correct reception of signals of the third slot S3 in the frame FR2 and subsequent frames FR3, FR4, ... of the signal of the frequency F2. On the other hand, when the mobile station 10 transmits a signal U2 to the base station B2 in the third slot S3 of the next frame FR2 after establishment of synchronization for reception, the base station B2 establishes synchronization for reception with respect to the signal U2. Consequently, the base station B2 correctly receives signals U3, U4, ... of the third slot S3 from the mobile station 10 in the next and subsequent frames FR3, FR4, FR5, ....
Incidentally, the base station B2 uses, for the establishment of synchronization, the signal U2 received from the mobile station 10 in the slot S3 of the frame FR2 but cannot receive it as a communication signal--this means the occurrence of a momentary cutoff of communication. Moreover, in the case where the base station B2 fails to establish synchronization with the signal U2 from the mobile station 10, it uses, for establishment of synchronization for reception, the signal U3 from the mobile station 10 in the slot S3 of the frame FR3, and hence cannot receive the signal U3 as a communication signal in the slot S3. In the above the mobile station 10 has been described to have established synchronization for reception, in the slot S3 of the frame FR1, with respect to the signal B1 from the base station B1 in the frame FR1, but if the mobile station fails to establish the synchronization, the mobile station 10 uses a signal B2 from the base station B2 in the next frame FR2 to establish synchronization for reception, and hence cannot receive the signal B2 as a communication signal.
As mentioned above, the conventional digital mobile communication system, which merely applies the TDMA techniques to the mobile communication system, also suffers from the problem of an instantaneous interruption of communication by the switching of the radio channel.
The above description has been given of the momentary interruption of communication which is caused by the switching of the radio channel when the mobile station moves from a certain zone to the adjoining one, but also in the case where communication of the mobile station with the base station in the same zone is degraded by interference of other signals such as an interference signal of the same frequency as that of the received signal or its multiple reflection, the communication is interrupted momentarily when the channel is switched to another frequency channel assigned to the same base station.
It is therefore an object of the present invention to provide a radio channel hand off method which solves the above-mentioned problem encountered in the digital mobile communication system employing the TDMA techniques, and hence precludes the possibility of such a momentary interruption of communication as described above.